The present invention relates to a force sensing probe of the type used with coordinate measuring machines (CMMs) and the like. Particularly but not exclusively the invention relates to the determination of the force exerted on a probe stylus when it is urged against a workpiece. Also the invention relates to improvements in the construction of such probes.
Profile measurement or scanning probes have generally an analogue output which increases as the deflection of the probe stylus increases and which is added or subtracted to or from known measurement machine readings. In this way an unknown profile can be followed and multiple machine readings can be used (adjusted by the probe stylus deviation) to determine the shape of the unknown profile. Analogue probes can be expensive.
A so-called hard probe, without sensing, can be used to make contact with a workpiece. Measurement of dimensions at the point of contact can be made by noting a machine""s reading with no significant deflection of the stylus. A refinement of this probe allows the contact forces between the stylus and the workpiece to be measured i.e. a so-called force sensing probe. A probe of this type is outlined in U.S. Pat. No. 5,611,147 (Faro) at column 16 lines 3-28 and FIG. 26G.
Referring to FIG. 1 of the present drawings there is shown an articulatable arm 8 having various articulatable elements which are intended to be manually manipulatable in the direction shown by the arrows. A detailed explanation of this arm is provided in U.S. Pat. No. 5,611,147 which is incorporated herein by reference.
In use the arm""s position is monitored by a system which determines the position of the stylus by monitoring the relative positions of all the articulatable elements. Measurements can be taken when the stylus is in contact with a workpiece to be measured (in this illustration a door 20).
Conventionally an operator touches the stylus against the workpiece and presses a button to take a stylus position reading. U.S. Pat. No. 5,611,147 at column 16 lines 3-28 and FIG. 26G describes a force sensing probe which automatically produces a xe2x80x9ctake a readingxe2x80x9d signal when sufficient contact force is exerted on the stylus.
Few probe constructional details are disclosed in U.S. Pat. No. 5,611,147.
According to one aspect the present invention provides a measurement probe comprising as stylus holder, a probe body and a force transmitting member connecting the holder to the body, the member having strain sensing elements associated therewith for measuring the strain exerted on the member, characterised in that the probe comprises a diaphragm in spaced parallel relationship with the member, further connecting the holder to the body.
Preferably the holder has a portion of reduced stiffness at or adjacent the member.
Preferably the diaphragm is planar. Preferably the member has three strain sensing elements arranged at 120xc2x0 intervals about a central region of the member. More preferably the intervals are formed by three spokes. Possibly a fourth reference element is provided mounted to a part of the member which is unstrained in use.
Alternatively the member may have six strain sensing elements. These elements may be arranged as two sets of three elements, each element of each set being positioned about a central region of the member and may be separated by an interval of approximately 120xc2x0. One set of elements may be positioned on three spokes extending radially from the central region.
U.S. Pat. No. 5,724,725 (Zeiss) describes a method of manually measuring a workpiece wherein only those measurements which fall within a range of stylus contact forces are validated. The forces are measured by monitoring deflections of various parts of the probe supporting structure but the forces within the probe itself are not measured. The system described in U.S. Pat. No. 5,724,745 may not determine accurately the contact forces on the stylus if any part other than handle (19 FIG. 1) is used to push the stylus (12) onto a workpiece.
According to a second independent aspect the invention provides a measurement probe to which a stylus is connectable for use with a coordinate determination device for determining the coordinates of points on a workpiece when contacted by the stylus, the probe comprising a stylus contact force sensing arrangement in use having a first state for allowing coordinate determination by the device and a second state for preventing coordinate determination by the device, characterised in that the first state occurs when contact force on the stylus is within a force range having an upper and lower limit and the second state occurs when contact force on the stylus is outside the force range.
Thereby the second state may occur when the contact force is either above the upper limit or below the lower limit of the force range.
Preferably the force sensing arrangement is disposed substantially within the probe.
Preferably the upper and lower limits are independently selectively adjustable.
The occurrence of the first or second states may be accompanied by an indication at the probe. More preferably the indication is the illumination of a light or the operation of an audible device. A different indication may be given for each of the first and second states.
According to a third independent aspect the invention provides a measurement probe comprising a force sensing member and a housing surrounding the member, a cover surrounding at least the portion of the housing that surrounds the member, the cover having a handgrip portion, the housing and the cover being separated.
Preferably the separation is an air gap, providing thermal insulation and protection of the member from manual force influences.
Preferably the handgrip portion comprises grooves in the cover and elastomeric rings disposed in the grooves.